LTE has 3 types of Resource Allocation Types - RAT0, RAT1, RAT2. Which one is the best?

Question

LTE has 3 types of Resource Allocation Types(RAT) - RAT0,RAT1,RAT2.

Q1: Among these three which one is the best?
Q2: In What basis, will RAT be chosen?,
Q3: Can we dynamically change the RAT over time?

Answer 1

Its all about trade off among flexibility vs complexity vs usage/wastage.
Pick which suits you best :-)

RAT is used to minimize control size information.(i.e DCIs)
Spec 36.213 has fixed mapping for DL DCIs and RAT used.

(1A,1B,1C,1D) : RAT 2
(1,2,2A,2B,2C) : RAT 0 OR RAT 1

RAT 0 : assign in RBGs and easy mapping but chances of wastage of RBs are more.
RAT 1 : This one really good , yet there is easy mapping (SUBSET) and assigns in RBs as well.
RAT 2(Localized or Distributed) : It(eNB MAC) assigns contiguous RBs .
If Distributed is selected , PHY may not keep it contiguous while transmitting else no change at MAC.

Hope this helps